The present invention relates to the communication of digital television signals, and more particularly to a waveform generator useful for providing a digital television signal with data of a type conventionally carried in the vertical blanking interval (VBI) of an analog television signal. Examples of such data, hereinafter referred to as "user data", include closed caption data (CC), vertical interval time code (VITC), non-realtime video data (e.g., vertical interval test signals--VITS), sampled video data, North American Basic Teletext Specification (NABTS), World System Teletext (WST), European Broadcast Union (EBU) data and Nielsen Automated Measurement of Lineup (AMOL) data.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast via a cable television network or by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have replaced analog phonograph records in the audio industry.
One way to transmit the compressed video data to a receiver is in the form of packets contained within a packetized data stream. Typically, packets carrying compressed video data are multiplexed with other packets, e.g., carrying corresponding audio data and control information necessary to reconstruct a television signal. One standard for transporting digital television signals in this manner is the MPEG-2 standard, details of which can be found in the International Organisation for Standardisation, ISO/IEC 13818-1, International Standard, Nov. 13, 1994 entitled "Generic Coding of Moving Pictures and Associated Audio: Systems," recommendation H.222.0, incorporated herein by reference. Further details of the video syntax and semantics for MPEG-2 video can be found in International Organisation for Standardisation, ISO/IEC 13818-2, International Standard, 1995 entitled "Generic Coding of Moving Pictures and Associated Audio: Video," recommendation H.262, also incorporated herein by reference.
Another standard for transporting digital television data in a packet stream is the Advanced Television Systems Committee (ATSC) Digital Television Standard A/53, approved on Apr. 12 and Sep. 15, 1995, incorporated herein by reference. The ATSC Digital Television Standard is based on the ISO/IEC MPEG-2 Video Standard, the Digital Audio Compression (AC-3) Standard, and the ISO/IEC MPEG-2 Systems Standard.
In the ATSC and MPEG-2 systems (and the similar DigiCipher.RTM. II system proprietary to General Instrument Corporation, the assignee hereof) a transport stream, or transport multiplex is made up of a contiguous set of fixed length packets. The video sequence is transported using a hierarchical structure in which a sequence header is followed by various extensions, user data, a group of pictures ("GOP") header, optional user data, a picture header, etc. The sequence header provides information for a sequence of pictures, which in general will include more than one GOP. This information includes, for example, horizontal and vertical size values, aspect ratio, frame and bit rate, and quantization parameters for the video data. A user data extension can also be included which, among other things, provides additional data for use by decoders. The DigiCipher.RTM. II standard provides for the transport of additional user data after the sequence header, in order to identify a DigiCipher.RTM. II signal and the use of any special video compression techniques used within a sequence, including DigiCipher.RTM. special prediction and block motion estimation.
In both the MPEG-2 and DigiCipher.RTM. II syntaxes, a sequence display extension containing, e.g., video format and color description information, is provided in addition to the sequence extension and user data. A subsequent group of pictures header provides, among other information, a time code. Thereafter, a picture header is provided which includes various information pertaining to a corresponding picture in a sequence of pictures to be displayed. A picture extension and, ultimately, the actual picture data to be decoded and reproduced for viewing, is then provided. It is noted that MPEG does not specify the order in which various extensions (such as the sequence display extension) or the user data must be transmitted beyond the fact that they must be after the sequence extension and before the GOP header (if provided) or the picture header. MPEG does not require GOP headers to be sent, and such headers may be bypassed in particular implementations.
In a practical transmission system it may be necessary to include additional data at different times for specific purposes, such as providing closed captioning, VITS, auxiliary real time video, Teletext, and AMOL data. Such additional data may be carried in the vertical blanking interval (VBI) portions of an analog television signal, and is referred to herein as "VBI user information", "user data", or "user information."
Many standards have been developed for services provided via waveforms carried in the VBI lines of analog and composite video. Digital video compression systems tend to employ algorithms optimized for the characteristics of two dimensional motion video. These algorithms are not generally well suited for the compression of video waveforms present in the VBI lines of analog video.
The character of VBI waveforms is very different compared to active video. Transmission without compression for these lines is very bandwidth intensive, such as sending 8 or 10 bit samples of 704 or 720 luminance and chrominance pixels. For example, 720 luminance and chrominance values at 8 bit resolution and 30 Hz requires 345,600 bps while the information conveyed by these lines only represents 480 bps for closed captions and 6720 bps for North American Basic Teletext Specification. As the transition to digital video proceeds, the demand for carriage and reconstruction of VBI services continues. Digital video distribution systems are expected to reconstruct the VBI as well as the active video, even when digital video compression techniques are employed. Thus, there is a need for algorithms, syntax and semantics specifically for the compression of VBI video lines that will allow an efficient and flexible alternative to developing VBI-waveform specific user data syntax and semantics.
It would be advantageous to provide a generic transport syntax and semantics for digital television data that would accommodate various types of VBI user information which may or may not be used at any given time. Such a scheme would enable the economical management of bandwidth while providing flexibility as to the transport of VBI user information.
It would be further advantageous to provide a waveform generator for providing digital waveforms that comply with the various VBI waveform standards, in response to the generic transport syntax. The present invention provides a generic waveform generator enjoying the aforementioned advantages.